"Though we acknowledged the sincere efforts of both Turkey and Brazil to find a solution regarding Iran's stand-off with the international community over its unclear program, the P-5 Plus 1 - which consists, of course, of Russia, China the United States, the U.K., France and Germany along with the High Representative of the EU - are proceeding to rally the international community on behalf of a strong sanctions resolution that will in our view send an unmistakable message about what is expected from Iran," she said.

Monday in Tehran, Iran said it was accepting a variant of a big-power, confidence-building proposal made last November, under which it would export much of its low-enriched uranium stockpile in return for higher-enriched fuel for a Tehran research reactor.

But critics of the deal, described by Brazilian President Luiz Inacio Lula da Silva as a victory of diplomacy, noted it did not include a commitment by Iran to stop enrichment, and that Iran would retain enough of its uranium stocks to potentially build a weapon.

Introducing Clinton at the hearing, Senate Foreign Relations Committee Chairman John Kerry said given Iran's continued enrichment drive since last November, its acceptance of the offer now is less significant.

"As we know, during the course of the months since that original deal was put on the table, Iran has gone from about 1,800 kilograms to 2,300 kilograms [of enriched uranium], and so it is not the same deal," said Kerry. "And it is our understanding that the potential for a breakout to one nuclear weapon would exist during the time of this swap, absent further ingredients of a deal."